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Data upon which the following estimates of incidence of adverse reactions are made are derived from experiences reported in the literature, unpublished clinical trials and voluntary reports since marketing of ZYLOPRIM (allopurinol) began. Past experience suggested that the most frequent event following the initiation of allopurinol treatment was an increase in acute attacks of gout (average 6% in early studies). An analysis of current usage suggests that the incidence of acute gouty attacks has diminished to less than 1%. The explanation for this decrease has not been determined but may be due in part to initiating therapy more gradually (see PRECAUTIONS and DOSAGE AND ADMINISTRATION).
The most frequent adverse reaction to ZYLOPRIM (allopurinol) is skin rash. Skin reactions can be severe and sometimes fatal. Therefore, treatment with ZYLOPRIM (allopurinol) should be discontinued immediately if a rash develops (see WARNINGS). Some patients with the most severe reaction also had fever, chills, arthralgias, cholestatic jaundice, eosinophilia and mild leukocytosis or leukopenia. Among 55 patients with gout treated with ZYLO-PRIM for 3 to 34 months (average greater than 1 year) and followed prospectively, Rundles observed that 3% of patients developed a type of drug reaction which was predominantly a pru-ritic maculopapular skin eruption, sometimes scaly or exfolia-tive. However, with current usage, skin reactions have been observed less frequently than 1%. The explanation for this decrease is not obvious. The incidence of skin rash may be increased in the presence of renal insufficiency. The frequency of skin rash among patients receiving ampicillin or amoxicillin concurrently with ZYLOPRIM (allopurinol) has been reported to be increased (see PRECAUTIONS).
Most Common Reactions* Probably Causally Related:
Metabolic and Nutritional: Acute attacks of gout.
Skin and Appendages: Rash, maculopapular rash.
*Early clinical studies and incidence rates from early clinical experience with ZYLOPRIM (allopurinol) suggested that these adverse reactions were found to occur at a rate of greater than 1%. The most frequent event observed was acute attacks of gout following the initiation of therapy. Analyses of current usage suggest that the incidence of these adverse reactions is now less than 1%. The explanation for this decrease has not been determined, but it may be due to following recommended usage (see ADVERSE REACTIONS introduction, INDICATIONS AND USAGE, PRECAUTIONS, and DOSAGE AND ADMINISTRATION).
Incidence Less Than 1% Probably Causally Related:
Body As a Whole: Ecchymosis, fever, headache.
Hemic and Lymphatic: Thrombocytopenia, eosinophilia, leukocytosis, leukopenia.
Musculoskeletal: Myopathy, arthralgias.
Skin and Appendages: Erythema multiforme exudativum (Stevens-Johnson syndrome), toxic epidermal necrolysis (Lyell's syndrome), hypersensitivity vasculitis, purpura, vesicular bullous dermatitis, exfoliative dermatitis, eczematoid dermatitis, pruritus, urticaria, alopecia, onycholysis, lichen planus.
Special Senses: Taste loss/perversion.
Incidence Less Than 1% Causal Relationship Unknown:
Body As a Whole: Malaise.
Hemic and Lymphatic: Aplastic anemia, agranulocytosis, eosinophilic fibrohistiocytic lesion of bone marrow, pancyto-penia, prothrombin decrease, anemia, hemolytic anemia, reticu-locytosis, lymphadenopathy, lymphocytosis.
Skin and Appendages: Furunculosis, facial edema, sweating, skin edema.
Read the Zyloprim (allopurinol) Side Effects Center for a complete guide to possible side effects
In patients receiving mercaptopurine or IMU-RAN (azathioprine), the concomitant administration of 300 to 600 mg of ZYLOPRIM (allopurinol) per day will require a reduction in dose to approximately one third to one fourth of the usual dose of mercaptopurine or azathioprine. Subsequent adjustment of doses of mercaptopurine or azathioprine should be made on the basis of therapeutic response and the appearance of toxic effects (see CLINICAL PHARMACOLOGY).
It has been reported that ZYLOPRIM (allopurinol) prolongs the half-life of the anticoagulant, dicumarol. The clinical basis of this drug interaction has not been established but should be noted when ZYLOPRIM (allopurinol) is given to patients already on dicumarol therapy.
Since the excretion of oxipurinol is similar to that of urate, uricosuric agents, which increase the excretion of urate, are also likely to increase the excretion of oxipurinol and thus lower the degree of inhibition of xanthine oxidase. The concomitant administration of uricosuric agents and ZYLOPRIM (allopurinol) has been associated with a decrease in the excretion of oxypurines (hypoxanthine and xanthine) and an increase in urinary uric acid excretion compared with that observed with ZYLOPRIM (allopurinol) alone. Although clinical evidence to date has not demonstrated renal precipitation of oxypurines in patients either on ZYLO-PRIM alone or in combination with uricosuric agents, the possibility should be kept in mind.
The reports that the concomitant use of ZYLOPRIM (allopurinol) and thi-azide diuretics may contribute to the enhancement of allopuri-nol toxicity in some patients have been reviewed in an attempt to establish a cause-and-effect relationship and a mechanism of causation. Review of these case reports indicates that the patients were mainly receiving thiazide diuretics for hypertension and that tests to rule out decreased renal function secondary to hypertensive nephropathy were not often performed. In those patients in whom renal insufficiency was documented, however, the recommendation to lower the dose of ZYLOPRIM (allopurinol) was not followed. Although a causal mechanism and a cause-and-effect relationship have not been established, current evidence suggests that renal function should be monitored in patients on thi-azide diuretics and ZYLOPRIM (allopurinol) even in the absence of renal failure, and dosage levels should be even more conservatively adjusted in those patients on such combined therapy if diminished renal function is detected.
An increase in the frequency of skin rash has been reported among patients receiving ampicillin or amoxicillin concurrently with ZYLOPRIM (allopurinol) compared to patients who are not receiving both drugs. The cause of the reported association has not been established.
Enhanced bone marrow suppression by cyclophosphamide and other cytotoxic agents has been reported among patients with neoplastic disease, except leukemia, in the presence of ZYLOPRIM (allopurinol) . However, in a well-controlled study of patients with lymphoma on combination therapy, ZYLOPRIM (allopurinol) did not increase the marrow toxicity of patients treated with cyclophos-phamide, doxorubicin, bleomycin, procarbazine, and/or mechlorethamine.
Tolbutamide's conversion to inactive metabolites has been shown to be catalyzed by xanthine oxidase from rat liver. The clinical significance, if any, of these observations is unknown.
Chlorpropamide's plasma half-life may be prolonged by ZYLOPRIM (allopurinol) , since ZYLOPRIM (allopurinol) and chlorpropamide may compete for excretion in the renal tubule. The risk of hypoglycemia secondary to this mechanism may be increased if ZYLOPRIM (allopurinol) and chlorpropamide are given concomitantly in the presence of renal insufficiency.
Rare reports indicate that cyclosporine levels may be increased during concomitant treatment with ZYLOPRIM (allopurinol) . Monitoring of cyclosporine levels and possible adjustment of cyclo-sporine dosage should be considered when these drugs are co-administered.
Drug/Laboratory Test Interactions: ZYLOPRIM (allopurinol) is not known to alter the accuracy of laboratory tests.
Read the Zyloprim Drug Interactions Center for a complete guide to possible interactions
Last reviewed on RxList: 10/11/2010
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